Method for decentralized synchronization in a self-organizing radio communication system

ABSTRACT

A method performs synchronization in an at least partly self-organizing radio communication system with a number of mobile stations which lie across an air interface within two-way radio range. At least some mobile stations from the number of mobile stations transmit synchronization sequences, by which a part or all the mobile stations of the number of mobile stations synchronize.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/EP2003/011962 filed on Oct. 28, 2003 and EuropeanApplication No. 02257475.0 filed on Oct. 28, 2002, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for synchronization in a radiocommunication system having a plurality of mobile stations, the radiocommunication system being at least partly self-organizing.

The invention also relates to a mobile station in a radio communicationsystem, the radio communication system being at least partlyself-organizing, and to a radio communication system.

Communication systems are extremely important in the field of businessas well as in private use. Significant efforts are being made to linkcable-based communication systems to radio communication systems. Theresulting hybrid systems lead to an increase in the number of availableservices as well as allowing greater flexibility in terms ofcommunication. Devices that can use different systems (multi-homing) aretherefore being developed.

In this context, great importance is attached to radio communicationsystems due to the mobility they offer to the subscribers.

In radio communication systems, information (e.g. voice, imageinformation, video information, SMS [Short Message Service] or otherdata) is transmitted between a sending station and a receiving station(base station or subscriber station) via a radio interface usingelectromagnetic waves. In this case, the emission of the electromagneticwaves takes place using carrier frequencies that lie in the frequencyband which is designated for the system concerned.

In the case of the established GSM (Global System for MobileCommunication) mobile radio system, frequencies at 900, 1800 and 1900MHz are used. These systems principally transfer voice, facsimile andSMS short messages (Short Message Service) as well as digital data.

For future mobile radio systems that use CDMA or TD/CDMA transmissionmethods, e.g. UMTS (Universal Mobile Telecommunication System) or otherthird-generation systems, frequencies in the frequency band of approx.2000 MHz are planned. These third-generation systems are being developedto meet the aims of worldwide radio coverage, a broad offering of datatransmission services and, most importantly, flexible management of thecapacity of the radio interface, which is the interface with the fewestresources in the case of radio communication systems. In the context ofthese radio communication systems, the flexible management of the radiointerface should primarily allow a subscriber station to send and/orreceive a large volume of data at high data speeds as required.

The access of stations to the shared radio resources of the transmissionmedium, e.g. time, frequency, throughput or space, is governed bymultiple access (MA) methods in these radio communication systems.

In the case of time division multiple access methods (TDMA), each sendand receive frequency band is divided into time slots, wherein one ormore cyclically repeated time slots are allocated to the stations. UsingTDMA, the radio resource of time is separated in a station-specificmanner.

In the case of frequency division multiple access methods (FDMA), thecomplete frequency domain is divided into narrow-band domains, whereinone or more narrow-band frequency domains are allocated to the stations.Using FDMA, the radio resource of frequency is separated in astation-specific manner.

In the case of code division multiple access (CDMA) methods, thethroughput/information which has to be transmitted is encoded in astation-specific manner by a spreading code which is formed of amultiplicity of individual so-called chips, whereby the throughput whichmust be transmitted is spread randomly over a wide frequency domain inaccordance with a code. The spreading codes which are used by differentstations within a cell/base station are mutually orthogonal oressentially orthogonal in each case, whereby a receiver recognizes thesignal throughput which is intended for the receiver and suppressesother signals. Using CDMA, the radio resource of throughput is separatedin a station-specific manner by spreading codes.

In the case of orthogonal frequency multiple access methods (OFDM), thedata is transferred in a broadband manner, wherein the frequency band isdivided into equidistant orthogonal subcarriers, such that thesimultaneous phase shifting of the subcarriers covers a two-dimensionaldata flow in the time-frequency domain. Using OFDM, the radio resourceof frequency is separated in a station-specific manner by orthogonalsubcarriers. The combined data symbols which are transferred on theorthogonal subcarriers during a time unit are called OFDM symbols.

The multiple access methods can be combined. Many radio communicationsystems therefore use a combination of the TDMA and FDMA methods,wherein each narrow-band frequency band is divided into time slots.

For the purpose of the aforementioned UMTS mobile radio system, adistinction is made between a so-called FDD (frequency division duplex)mode and a TDD (time division duplex) mode. In particular, the TDD modeis characterized in that a shared frequency band is used for the signaltransmission in both uplink (UL) direction and in downlink (DL)direction, while the FDD mode uses a different frequency band for thetwo transmission directions in each case.

In radio communication connections of the second and/or thirdgeneration, information can be transmitted in a circuit-switched (CS) orpacket-switched (PS) manner.

The connection between the individual stations takes place via a radiocommunication interface (air interface). Base station and radio networkcontroller are usually components of a base station subsystem (RNS radionetwork subsystem). A cellular radio communication system normallyincludes a plurality of base station subsystems which are connected to acore network (CN). In this case, the radio network controller of thebase station subsystem is usually connected to an access facility of thecore network.

In addition to these hierarchically organized cellular radiocommunication systems, self-organizing wireless radio communicationsystems—e.g. so-called ad-hoc systems—are becoming increasinglyimportant, this applying also in the context of cellular radiocommunication systems.

Self-organizing radio communication systems generally also allow thedirect communication between mobile terminals, and need not have acentral entity which controls the access to the transmission medium.

Self-organizing radio communication systems make it possible for datapackets to be exchanged directly between moving radio stations withoutthe involvement of base stations. Consequently, an infrastructure in theform of base stations within a cellular structure is not required insuch a radio network. Instead, data packets can be exchanged betweenmoving radio stations which are within radio range of each other. Inorder to allow the exchange of data packets in principle, asynchronization is required between the radio stations which are usuallymoving. In the case of a wireless transmission via electronic waves,this means e.g. the balancing of carrier frequency (frequencysynchronization) and time slot pattern (time synchronization).

Various solutions are conceivable for the synchronization in mobileradio data networks. For example, the mobile stations can have a sharedreference which is transmitted e.g. via GPS. The system thereforeincludes globally known time information which all mobile stations canfollow (e.g. VDL Mode 4, or WO 93/01576, “A Position IndicatingSystem”). This method is disadvantageous firstly because all mobilestations must have a cost-intensive GPS receiver. Secondly, thereception of GPS signals e.g. in buildings is not always guaranteed.Other systems such as TETRA, for example, support the selection of amaster which assumes the function of a ‘clock signal generator’ for thefrequency domain that is assigned to it. However, such methods precludea high granularity in relation to the time (TDMA) and/or the code(CDMA). An FDMA component is preferably used for separating thesubscribers in this case. A third group of systems, such as e.g.IEEE802.11, operate without a shared time slot pattern. The mobilestations synchronize themselves by a one-shot synchronization on thebasis of the data burst which is received in each case. Reservation ofresources in the form of time slots for ensuring the QoS is no longerpossible in this case, however.

SUMMARY OF THE INVENTION

The inventors propose a method, a mobile station and a radiocommunication system of the type cited at the beginning, which allowtime-relative synchronization between moving radio stations for aself-organizing radio data network, without the presence of a cellularinfrastructure being necessary in order to achieve this. Thesynchronization should not be dependent on GPS and should be decentrallyorganizable. It should nonetheless be possible to support a framestructure in a network topology which varies significantly over time,since synchronization should be possible in the case of highly mobilesubscribers in particular, i.e. where there is significant fluctuationin the network topology (e.g. in the case of mobile stations in movingvehicles, cf. FIG. 1). In a further step, consideration should also begiven to the merging of asynchronously active clusters with regard tosynchronization, wherein mobile stations that are situated within areciprocal radio range are designated as clusters.

At least some mobile stations from the number of mobile stations willtransmit synchronization sequences, with reference to which some or allof the mobile stations from the number of mobile stations willsynchronize themselves.

As a result of the independence of the synchronization from the cellularinfrastructure, and in particular from base stations, thesynchronization takes place decentrally. The subscriber stations can bebut do not have to be mobile. These subscriber stations are designatedas mobile stations hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1: shows a network structure of a mobile self-organizing radio datanetwork,

FIG. 2: shows a first example for marking the synchronization sequencefor frame synchronization,

FIG. 3: shows a second example for marking the synchronization sequencefor frame synchronization,

FIG. 4: shows a frame structure for the UTRA-TDD mode (Low Chip Rate),

FIG. 5:—PartA an example of a sequential decentral synchronization,—Part B an example of a joint decentral synchronization,

FIG. 6: shows an example of two asynchronous clusters,

FIG. 7: shows an illustration of an active guard zone and a passiveguard zone,

FIG. 8: shows an illustration of an active guard zone and theinterference ranges for three mobile stations N₄, N₅ and N₆,

FIG. 9: shows an illustration of a passive guard zone for three mobilestations N₄, N₅ and N₆,

FIG. 10: shows a frame structure for the UTRA-TDD mode (Low Chip Rate).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The invention is particularly suitable for TDD/TDMA-basedtechnologies—as are currently being discussed for the next generation ofmobile communications. The method can be used advantageously in avariant of the (current) 3rd generation of mobile communications, forexample, since the decentrally organized synchronization for highlymobile radio data networks can be implemented for the Low Chip Rate(LCR) variant of UTRA TDD. It is easy to implement ported versions ofthe algorithms on TSM or HCR. In addition, the application can also beused on other time-slot-oriented access systems, e.g. DECT

In self-organizing radio networks having centrally organizedsynchronization, a mobile station assumes—within a cluster—the functionof the clock signal generator. This role can be established at thebeginning of the network construction. However, it can also be oflimited time duration. Protocol mechanisms which organize the selectionof the corresponding mobile stations are known (cf. e.g. HIPERLAN2).

In the case of decentrally organized synchronization, the function ofthe clock signal generator is not assumed by a single mobile station butby a subset of all participating mobile stations. In the extreme case,it is even possible to use all mobile stations for maintaining thesynchronization.

These mobile stations also transmit synchronization sequences inaddition to the actual payload data. The synchronization sequences canbe part of an information-carrying data packet in this case. However,they can also be supplied to the radio network separately by asynchronization channel which is dedicated, i.e. separate from thetransmission of payload data, the synchronization channel being definedon the basis of frequency, time and/or code multiplexing.

Synchronizing mobile stations detect the synchronization positionsT_(SYNC,i) of the other mobile stations and derive their ownsynchronization position from these. The quality of the individualdetected synchronization positions—which can be derived e.g. from theirreceived signal strength—can be taken into consideration in this case,as can the preceding synchronization position of the synchronizingmobile station.

The following relationship can be established for the time-basedsynchronization position T_(SYNC):

${T_{SYNC} = {{\alpha \cdot T_{{SYNC},{old}}} + {\frac{1 - \alpha}{\sum\limits_{i}g_{i}}{\sum\limits_{i}{g_{i} \cdot T_{{SYNC},i}}}}}};{0 \leq \alpha \leq 1}$

In this case, a is a weighting factor for the preceding synchronizationposition T_(SYNCold) of the synchronizing mobile station. Differentstrategies exist for the weighting g_(i) of the currently detectedsynchronization positions of the other mobiles i. Two are listed belowas examples:

$\quad\begin{matrix}\left. 1. \right) & \begin{matrix}{{detection}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{maximum}\text{:}} & {g_{i} = \begin{Bmatrix}1 & {{for}\mspace{14mu}{\max.\mspace{14mu}{recv}.\mspace{14mu}{level}}} \\0 & {otherwise}\end{Bmatrix}}\end{matrix} \\\left. 2. \right) & {{weighting}\mspace{14mu}{using}\mspace{14mu}{the}\mspace{14mu}{receive}\mspace{14mu}{{level}.}}\end{matrix}$

It is clear that the consideration of the preceding synchronization timepoints is particularly important for the convergence of the decentralsynchronization, and is therefore preferably used in combination withsynchronization positions of the other mobiles for the specification.The estimated value can be ‘continuously’ improved in this way.

Since the synchronization time point of a mobile station is generallyderived from a plurality of references, wherein the signal propagationtimes can vary significantly due to the different distances between theindividual mobiles, the variance of the synchronization position ispossibly greater in comparison with synchronization in a centrallyorganized network (e.g. with a base station). This can be taken intoconsideration when dimensioning the corresponding guard periods. In thecase of a range of 1 km, for example, an additional tolerance of up to 3μs is generated by the differences in propagation times alone, and hasto be compensated.

A plurality of embodiments are described below:

A. The transmission of the synchronization data can take place in thesame burst which carries the data. The position (e.g. as pre-amble ormid-amble) of the synchronization data which relates to the actual datasequence is irrelevant in this case.

B. The method is not based on a joint transmission of synchronizationdata and the actual data sequence. The synchronization data canoptionally also be transmitted via a further burst, which is separatedfrom the actual data burst by a CDMA, TDMA or even an FDMA component. Itis critical solely that the relative position of these bursts to theactual data burst must be established unambiguously.

C. The cyclical (not necessarily periodic) transmission of thesynchronization sequence is significant for maintenance of thesynchronization. One, a plurality or even all mobiles must ensure thatthis ‘service’ of the air interface is available. This applies inparticular if none of the participating mobiles transmits payload data.The cyclical transmission of a burst—also referred to subsequently as abeacon—which inter alia also carries the synchronization string isextremely advantageous both for the decentral synchronization inaccordance with the method which is described here and for theorganization of the self-organizing network, e.g. for identifying theneighbors who are situated within the radio range and for updating the‘neighbor list’.

D. Each mobile station derives its own reference clock from thesynchronization signals of the mobile stations which are situated withintheir synchronization range. The quality of these references can varysignificantly. While one of the mobile stations also uses a GPS signalas a reference, another can receive its reference clock exclusively fromthe received signals of the other mobile stations. In order to improvethe synchronization, a degree for the quality of the reference can bespecified in the beacon, for example, the degree then being taken intoconsideration by a corresponding weighting when the optimal samplinginstant is calculated.

E. In the case of access methods which combine a plurality of time slotsin a frame, or even combine a plurality of frames in so-calledsuperframes, it is necessary to define mechanisms which support framesynchronization. It is appropriate here to mark the relevant time slots,so that the position in the relevant frame can be inferred from themarking.

A simple possibility relates to, for example, using a differentsynchronization sequence for the first time slot as shown in Example 1in FIG. 2. FIG. 2 shows the marking of the synchronization sequence forframe synchronization in Example 1. However, the relatively longduration of the detection of the start of the frame is disadvantageousin this method. In the least favorable case, it is necessary to wait forthe complete frame duration until the corresponding sequence whichdefines the frame is repeated (assuming that provision has been made forat least one of the subscribers to generate a beacon in the first timeslot). Example 2 in FIG. 3 shows a faster possibility for framesynchronization. FIG. 3 shows the marking of the synchronizationsequence for frame synchronization in Example 2. Here thesynchronization string is always dependent on the location in the frame,i.e. a specific synchronization string (or a set of specificsynchronization strings) is assigned to each time slot. Therefore thetime slot synchronization also inherently supplies the framesynchronization. However, the high numerical effort is disadvantageousin this case, since a separate correlator must be provided for eachindividual synchronization string.

Sequential Synchronization—Joint Synchronization:

The decentral synchronization is characterized in that thesynchronization sequences can be sent from a plurality of mobilestations rather than from a single mobile station. In principle, thesynchronization sequences of the different mobile stations can occupy adifferent or identical radio resource (which is determined by frequencyband, time slot and/or code). A distinction is therefore made betweentwo types of decentral synchronization in the context of thisdiscussion:

Sequential Synchronization

Joint Synchronization

For clarification, it is relevant to explain a decentral synchronizationfor both modes on the basis of the frame structure for the UTRA-TDD mode(Low Chip Rate), the frame structure being defined by the 3GPP. This isoutlined in FIG. 4 [3GPP TS 25.221 V4.1.0].

The following must also be specified for FIG. 4:

Time slot#n (n from 0 to 6): the n^(th) traffic time slot, 864 chipsduration;

DwPTS: downlink pilot time slot, 96 chips duration;

UpPTS: uplink pilot time slot, 160 chips duration;

GP: main guard period for TDD operation, 96 chips duration;

The selected frame structure is also valid for TSM. Porting to the HighChip Rate variant of UTRA-TDD is possible without difficulty.

Sequential Synchronization:

The frame structure of UTRA-TDD has been optimized for operation incellular networks. Minor modifications are required for operation in aself-organizing radio network. Inter alia, in order to solve the problemof power impairment, it is proposed that only one mobile station beallowed to start transmit operation within a time slot. The differentcodes, of which there are up to 16, are then used for the purpose ofaddressing different receive-mobiles. Since operation is continuously ina type of ‘downlink mode’, it is possible to dispense with differentmid-ambles within a time slot, since each of the received mobiles isonly concerned with the estimation of a single channel. On the basis ofthe correlation to the characteristic mid-amble of the relevant timeslot, the timing of the relevant mobile can be determined in relation tothe internal timing. The notification relating to the detectedsynchronization positions then specifies a degree for the extent towhich the internal ‘time slot pattern’ must be adjusted. In order toreduce effort, it is possible to operate with the same mid-amble in alltime slots. However, it is necessary to mark a slot especially for theframe synchronization, e.g. by identifying a special synchronizationsequence for this slot. In this case, it must also be ensured that thisslot is always used by a mobile station, since otherwise the framesynchronization cannot be maintained.

Joint Synchronization:

In this case, in addition to the actual data-carrying burst, some of themobiles send the same synchronization sequence/beacon at the same timein a special time slot. This greatly simplifies the effort involved inthe synchronization.

The frame synchronization is an implicit element of the algorithm.

Resource-intensive averaging for locating the internal Sync.Position isomitted. The averaging takes place to a certain extent on thetransmission medium by virtue of the superimposition of the signalswhich carry the synchronization sequences.

Except for the fact that the mobile occasionally emits the Sync.Sequenceitself, the synchronization mechanism is completely identical to theoperation in the cellular case.

In the case of UTRA-TDD LCR, two special synchronization time slots areavailable. Both could be used effectively in the case of JointSynchronization. One time slot is used for receiving the synchronizationstring of the surrounding mobiles, while the other is used for sendingan internal synchronization string. All mobiles therefore transmit theirsynchronization string once in each frame and are able to synchronizewith their environment once at the same time. In the case of thesynchronization of a mobile to an already existing cluster, it wouldhowever be possible—as an exception to this rule—to operate bothsynchronization time slots in the receive mode. In order to distinguishbetween the synchronization time slots, a different synchronizationsequence is assigned to the first and second time slot. Each mobileshould allocate the transmission of its synchronization string to thetime slot which has the lower received power, thereby ensuring anapproximately uniform assignment of the mobiles to both time slots. Inparticular, the second active mobile is allocated to the unoccupied timeslot when setting up the cluster.

Reference is made to FIG. 5 for clarification of Sequential DecentralSynchronization and Joint Decentral Synchronization. The upper part FIG.5A of FIG. 5 illustrates the Sequential Decentral Synchronization andthe lower part FIG. 5B illustrates the Joint Decentral Synchronization.

The synchronization of asynchronous clusters/stations is consideredbelow. The principle of the guard zone is applied in this case.

FIG. 6 illustrates one of the essential challenges presented by thesynchronization in mobile self-organizing networks. In this case, 2clusters (each having 3 stations) are set up independently of each otherand can be operated asynchronously in relation to each other due totheir distance (both clusters lie outside of their reciprocal radioranges). Without a reference, such as e.g. GPS or the base station of amobile radio system, it is not possible to guarantee a synchronism ofthe two clusters. A method is described which achieves a ‘local’balancing of the synchronization parameters—particularly in the case ofclusters that ‘merge’—before there is any exchange of data between themobiles of the different clusters.

The solution which is indicated applies to self-organizing radionetworks having a centrally organized synchronization, but is alsoindependent thereof.

Radio Data Range and Synchronization Range:

The radio data range is defined in this case as the range within which apotential receiver is ‘only just’ able to guarantee a specified BER. Thesynchronization range is correspondingly defined as the range withinwhich the correct detection of the synchronization parameters, such ase.g. the time slice, can be guaranteed with a specified probability by apotential receiver.

Guard Zone:

The synchronization range of a station should be greater than theresulting range of the payload data (“radio data range”). The overreachof the synchronization information in this case defines the so-calledguard zone which can be used advantageously in order to achieve a localsynchronism of specified system parameters before the data exchangebetween the stations of the same cluster is significantly disrupted bythe transmissions of one or more stations of the approaching secondcluster.

FIG. 7 illustrates the principle of an active guard zone on one side anda passive guard zone on the other side.

Depending on whether the referenced station functions as a sender or areceiver, the guard zone is the to be active or passive respectively. Inthe first case, the guard zone ensures that all stations within theradio data range receive the data which is sent by the station N1; inthe second case, that all stations within the radio data range candeliver the data to the station N1 without the possibility of anasynchronously operating second cluster resulting in interference.

The objective of a greater synchronization range relative to the radiodata range can be achieved technically by the following method (appliedto the synchronization string):

-   -   greater send power (position required in a separate frequency        band)    -   lower modulation index    -   greater spreading factor when using band spreading techniques    -   greater receiver sensitivity    -   (optional) specification of a minimal required receive level for        the data detection.

Illustration of the guard zone with reference to the example of UTRA TDDLCR:

The following explanations are intended to consider in greater detailthe requirements relating to an active or passive guard zone for thecase of a self-organizing network on the basis of UTRA TDD LCR. Thefollowing assumptions are made:

-   -   the send power S of all stations is the same (UE class 2: 250        mW: 24 dBm)    -   the send power of data burst and synchronization burst are the        same    -   the maximal spreading factor for the data is 16; the maximal        spreading factor for the synchronization is 144    -   the signal-noise ratio (SNR) . . .        -   for a successful synchronization having a probability of 95%            is δ_(S)=−7.0 dB        -   for the data detection δ_(D)—a packet error rate of <10⁻²            should be guaranteed—is maximally    -   δ_(D)=7 dB, thereby producing a ratio of Δδ=δ_(D)−δ_(S)=14 dB.    -   the receiver sensitivity E_(D0) for the data according to the        standard is E_(D0)=−105 dBm.

The receiver sensitivity for the synchronization is Δδ more sensitivethan E_(D0) and is therefore E_(S)=E_(D0)−Δδ.

In order to reduce the data range, the required receive level for thedata detection E_(D) can (optionally) be raised by ε_(D)>0 dB, i.e.E_(D)=E_(D0)+ε_(D)=−105 dBm+ε_(D). Two examples are shown below, inwhich the raising of the receive level is necessary in order to maintainthe guard zone in the first example, and in which the raising of thelevel is forgone and therefore a greater range can be achieved in thesecond example.

The synchronization range is determined by the difference between sendlevel and receiver sensitivity E_(s), the Link Budget for the Sync. istherefore derived asξ_(S) =S−E _(S) =S−E _(D0)+Δδ=129 dB+Δδ.

It applies correspondingly for the data rangeξ_(D) =S−E _(D) =S−E _(D0)−ε_(D)=ξ_(S)−Δδ−ε_(D)=129 dB−ε_(D).

Reference is also made to the illustration in FIG. 8.

The requirements relating to an active guard zone are now brieflyexplained with reference to the above diagram. As a result of thetransmission of a data burst by the station N₁, both the data rangeand—due to the concurrent emission of the mid-amble—the Sync. range andtherefore the guard zone (for the station N₁) are determined. A stationN₂ lies in the radio data range of N₁. The interference power of apotential noise source (station N₄) should be δ_(D) lower than thereceive power of the data packet which is transmitted by the station N₁.The Path Loss between receive station N₂ and potential noise source N₄should be δ_(D)+ξ_(D) accordingly. Due to the different propagationpaths between N₁ and N₂ or between N₄ and N₂, the synchronization rangemust be at least ξ_(S)=2ξ_(D)+δ_(D). Therefore the necessary level forthe data reception must be raised by ε_(D)=0.5 (S−E_(D0)+δ_(S))=61 dB toE_(D)=−44 dBm. Assuming a free-space attenuation ofρ/dB=32.44+20 log₁₀(r/km)+20 log₁₀(f _(C)/MHz)

a data range of <50 m is produced.

The illustration in FIG. 9 shows the following:

In contrast with the active guard zone, the passive guard zone does notprotect the data reception of a third station which is situated withinthe data range of N₁, but instead guarantees that a data transmission ofstations such as N₂, N₃ to N₁ is not significantly disrupted by theapproaching second cluster comprising the nodes N₄, N₅, N₆. This greatlysimplifies the requirements relating to the guard zone. In this case,the synchronization range need only guarantee a distance ofξ_(S)=ξ_(D)+δ_(D).

In terms of raising the receiver sensitivity to a minimal receive levelfor the data detection, it therefore applies that ε_(D)=δ_(S)<0 dB. Araising of the receiver sensitivity is therefore unnecessary. Thereremains a reserve of 7 dB. The achievable data ranges are clearlygreater than 10 km.

The following statements apply in this context:

It is possible to achieve significantly greater ranges using a passiveguard zone than can be achieved using an active guard zone.

The effort for the synchronization is significantly greater in the caseof the passive guard zone. The passive guard zone must ‘protect’ apotential sender, and therefore the Sync sequence of the sender must becontinuously/cyclically transmitted. This applies in principle for allstations of a cluster. By contrast, the active guard zone need only beset up for the relevant station shortly before the transmission. Inorder to ensure the efficient utilization of the radio resources, thepassive guard zone should be combined with Joint Synchronization, inwhich case all mobiles of a cluster jointly occupy one resource only.

If different send powers are used when working, it is necessary eitherto switch to a separate frequency band for the transmission of the Sync.sequence (and operate there with the maximum send power) or take intoconsideration the difference between maximal and minimal send power inthe power budget.

In mobile radio data networks, the merging of two clusters which havebeen independently synchronized and are therefore usually asynchronousplaces particularly high demands on the decentral synchronization. Thesynchronization range of a station should be greater than the resultingrange of the payload data. The overreach of the synchronizationinformation in this case defines the so-called guard zone, which can beused advantageously in order to achieve a local synchronism of specificsystem parameters before the exchange of data between the stations ofthe same cluster is significantly disrupted by the transmissions of oneor more stations of the approaching second cluster. The objective of agreater synchronization range relative to the radio data range can beachieved technically by the following method (applied to thesynchronization string):

-   -   greater send power    -   lower modulation index    -   greater spreading factor when using band spreading techniques    -   greater receiver sensitivity    -   (optional) specification of a minimal required receive level for        the data detection.

A further embodiment is proposed in the following:

Decentral slot synchronization for self-organizing radio data networkson the basis of the slotted ALOHA method.

In a radio system according to the pure ALOHA method, each subscribersends its data immediately after generation thereof in data packets offixed length. Since the current occupancy of the radio channel is notchecked before the transmission, collisions with the emissions of othersubscribers can easily occur. Two data packets are lost if they collide,i.e. if they overlap even slightly in time.

A clear improvement in the number of successful transmissions can beachieved if the subscribers are only allowed to send at specific timepoints. This modification of the pure ALOHA method is called slottedALOHA. In comparison with pure ALOHA, the time period in which two datapackets can collide is halved for slotted ALOHA.

In the case of slotted ALOHA, a burst which is transmitted within a timeslot could have the structure that is shown in FIG. 10, for example. Aswell as the actual data sequence, the burst contains at least oneadditional sequence which is known to both the sender and the receiverand can be used for both the synchronization and the channel estimation.Depending on the arrangement within the burst, the terms pre-amble ormid-amble are also used. The so-called Guard Period (GP) is used tocompensate for runtime differences and reference clock tolerances of thesubscribers. Use is normally made of signal spreading techniques for thesynchronization. The decentral slot synchronization which is presentedunder Point 3 can therefore be used advantageously for thesynchronization of the time slots.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention covered by the claims which may include thephrase “at least one of A, B and C” or a similar phrase as analternative expression that means one or more of A, B and C may be used,contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed.Cir. 2004).

1. A method for synchronization of mobile stations in a radiocommunication system that is at least partly self-organizing and hasmobile stations which are situated in reciprocal radio range via an airinterface, comprising: transmitting synchronization sequences from atleast some of the mobile stations; using the synchronization sequencesfrom other mobile stations so that each mobile station can synchronizeitself; and for at least one of the mobile stations, transmittingpayload data with a range that is less than a range for synchronizationsequences transmitted by the mobile station.
 2. The method according toclaim 1, wherein the synchronization sequences are part of a data packetwhich carries information.
 3. The method according to claim 1, whereinthe synchronization sequences are transmitted on a dedicatedsynchronization channel.
 4. The method according to claim 1, whereinsynchronization sequences are transmitted in the same burst which alsocarries the payload data.
 5. The method according to claim 1, whereinthe synchronization sequences are transmitted cyclically orperiodically.
 6. The method according to claim 1, wherein the mobilestation uses the synchronization sequences to synchronize time slots. 7.The method according to claim 1, wherein only one mobile station startsa transmit operation within each time slot.
 8. The method according toclaim 1, wherein in order for a mobile station to synchronize itself,the mobile stations derives an internal synchronization position, theinternal synchronization position being derived from synchronizationpositions detected from the other mobile stations.
 9. The methodaccording to claim 8, wherein when deriving the internal synchronizationposition, the mobile station takes into consideration a quality level ofeach of the detected synchronization positions and/or its precedingsynchronization position.
 10. The method according to claim 1, whereinthe synchronization sequences are transmitted via bursts which areseparate from payload data bursts.
 11. The method according to claim 10,wherein the synchronization sequences are transmitted cyclically orperiodically.
 12. The method according to claim 11, wherein the mobilestations transmit a quality level of their synchronization together withthe synchronization sequences in order to improve synchronization. 13.The method according to claim 12, wherein the synchronization sequencesare transmitted via bursts which are separate from payload data bursts.14. The method according to claim 13, wherein the mobile station usesthe synchronization sequences to synchronize time slots.
 15. The methodaccording to claim 14, wherein only one mobile station starts a transmitoperation within each time slot.
 16. The method according to claim 1,wherein the mobile stations transmit a quality level of theirsynchronization together with the synchronization sequences in order toimprove synchronization.
 17. The method according to claim 16, whereinthe synchronization sequences are transmitted via bursts which areseparate from payload data bursts.
 18. A mobile station for a radiocommunication system which is at least partly self-organizing,comprising: a transmitter to: send synchronization sequences withreference to which other mobile stations can synchronize themselves, andsend payload data with a range that is less than a range for thesynchronization sequences sent by the mobile station.
 19. The mobilestation according to claim 18, further comprising: a receiver to receivesynchronization sequences from other mobile stations.
 20. A radiocommunication system that is at least partly self-organizing,comprising: a plurality of mobile stations each having a transmitter to:send synchronization sequences with reference to which other mobilestations can synchronize themselves, and send payload data with a rangethat is less than a range for the synchronization sequences sent by themobile station.
 21. A method for synchronization of mobile stations in aradio communication system that is at least partly self-organizing andhas mobile stations which are situated in reciprocal radio range via anair interface, comprising: transmitting synchronization sequences fromat least some of the mobile stations; using the synchronizationsequences from other mobile stations, such that each mobile station cansynchronize itself; and for at least one of the mobile stations,transmitting payload data with a range that is less than a range forsynchronization sequences transmitted by the mobile station, therebydefining a guard zone as the region in between the range of the payloaddata and the range of the synchronization sequences, whereinsynchronization is performed without GPS.